1. Field of the Invention
This invention relates to power transmission systems for working machines such as a snow removing machine and a power tiller and, more particularly, to an improvement over a torque limiter of such power transmission systems.
2. Description of the Related Art
An auger type snow removing machine is of the type wherein a spiral blade, known as an auger, collects snow and the collected snow is delivered to a blower for discharge. The auger is driven by an engine or electric motor (hereinafter referred to as a prime mover) via a power transmission system. When operating the auger, the auger tends to bite a lump of ice or stones (hereinafter referred to as foreign matters) and the power transmission system suffers from an overload, resulting in damages to the power transmission system. To solve this problem, it has heretofore been proposed to provide a torque limiter in the power transmission system so as to permit slippage in the power transmission system when it is subjected to a torque beyond a given level. One of these examples is disclosed in Japanese Utility Model Laid-Open Publication No. HEI-2-112715.
The auger type snow removing machine usually includes an input shaft driven by the prime mover and an auger drive shaft to which drive power is transmitted from the prime mover through a power transmitting mechanism. The torque limiter forming part of the power transmitting mechanism includes a worm wheel located at a central portion of the auger drive shaft. The central portion of the auger drive shaft has an outer periphery treated with sulphurizing or carburizing processes such that when the auger drive shaft is subjected to the overload, the worm wheel tends to rotate in an idling condition relative to the auger drive shaft.
The torque limiter includes the worm wheel and auger drive shaft which are assembled by initially accommodating the worm wheel in a gear case and subsequently press fitting the auger drive shaft to the worm wheel. This assembling method is disadvantageous in that the worm wheel is firmly retained to bear the auger drive shaft that is press fitted to the worm wheel, providing a difficult assembling process. To solve this problem, it has been the usual practice to employ an improved assembling process wherein the worm wheel is initially press fitted to the auger drive shaft to provide a unitary structure and subsequently the unitary structure is placed in the gear case. However, this assembling process requires that the gear case be divided into two halves on a line perpendicular to the auger drive shaft, resulting in a structurally complicated gear case.
Japanese Utility Model Laid-Open Publication No. SHO-58-157029 discloses a clutch mechanism suited for use on a small-sized tiller wherein an engine output shaft is connected to a drive shaft via a clutch. Owing to the clutch mechanism, a clutch housing and the engine are located at a low level near the ground such that the small-sized tiller has a lowered center of gravity. The clutch mechanism also has an overload protection function. That is, the clutch mechanism includes a spring, the force of which is adjusted to a desired value to provide the overload protection function. A technique employing a spring to effect overload protection is disclosed in Japanese Utility Model Post-Exam Publication No. SHO-39-4607 entitled xe2x80x9cTilling Shaft Load Control Device for Tillerxe2x80x9d wherein a load control device is employed to prevent tilling craws of the tiller from being ruptured.
In the clutch mechanism of Japanese Utility Model Laid-Open Publication No. SHO-58-157029, the clutch is complicated in structure and hence a torque limiter becomes complicated in structure. Being comprised of the clutch, a clutch release arm and an arm shaft, the clutch mechanism is large in size. Thus, the whole arrangement for accommodating the clutch mechanism is made large sized in lateral direction and has an increased weight, thereby deteriorating maneuverability of the tiller.
It is therefore a first object of the present invention to provide a power transmission system for a working machine having a torque limiter which is easy to assemble.
Another object of the present invention is to provide a power transmission system for a working machine having a torque limiter which is simple in structure, small in lateral direction and light in weight.
According to an aspect of the present invention, there is provided a power transmission system for a working machine, wherein drive power is transmitted from a prime mover to a work-tool holding drive shaft via a reduction mechanism for driving working members mounted on the drive shaft in its working condition. The reduction mechanism includes an input shaft, a first gear mounted on the input shaft, an output shaft, a second gear mounted on the output shaft and meshing with the first gear, and a gear case for accommodating the first and second gears. The second gear includes a torque limiter composed of a gear portion and a cylindrical boss segment press fitted to the gear portion.
The second gear of the reduction mechanism forms part of the torque limiter wherein the cylindrical boss segment is press fitted to the gear portion and, the reduction mechanism employing the torque limiter is simple in structure, easy to assemble, short in assembling time and low in manufacturing cost. Since the torque limiter has a lateral dimension selected to be in a range corresponding to a press fitting stroke of associated parts, the torque limiter has a reduced size and the reduction mechanism is small in size and light in weight.
In one preferred embodiment, the reduction mechanism forming part of the power transmission system comprises a worm reduction mechanism, wherein the first gear is a worm gear and the second gear is a gear wheel.
At least one of the contact surface of the boss segment and the contact surface of the gear segment may be treated with sulphurizing process such that slipping surfaces are prevented from being worn and the life of the torque limiter is prolonged.
Desirably, the input shaft is rotatably supported by two bearings, one of which is an angular type bearing. For example, in the event that the input shaft includes a bevel gear, the input shaft is subjected to not only a thrust load but also a radial load. With the use of the angular type bearing, both loads are born by the angular type bearing and the number of bearings can be reduced.
It is preferred that one end of the boss segment has an annular flange and the gear segment has an annular recess to receive the annular flange of the boss segment. When the gear segment is moved in a thrust direction relative to the gear segment, the gear segment is prevented from moving an excessive amount by the annular flange of the boss segment. Also, the boss segment can be reliably positioned relative to the gear segment during press fitting of the boss segment to the gear segment by fitting the annular flange of the boss segment to the annular recess of the gear segment and an assembling process of the wheel as a unitary structure can be simplified. For example, in the event that the second gear is a bevel gear, positioning of the associated parts may be simplified during press fitting thereof by fitting the annular flange of the boss segment to the annular recess of the gear segment, thus enabling easy assemblage of the bevel gear into a unitary structure.